Im also having this problem.
Also when the gunner is turned-in the First-Person-View ("Camera") is at the soldiers feet.
If you or anyone else has found a solution to this problem then please post it.
Thanks.
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+Update:
So the inGunnerMayFire as well as the outGunnerMayFire set to 0 are bugging the view.
It does not remove the ability to shoot turned in / out.
Those two parameters are also not noted on the "Arma 3 Car Config Guidelines".
Has someone found a solution to prevent the gunner from shooting while turned-in?